feat: tuple syntax cutover — Tuple(...) type + .(...) value

Replace the bare-paren tuple grammar with explicit, position-unambiguous
forms, mirroring how structs work:

  type     `(A, B)`        -> `Tuple(A, B)`          (named keeps `:`)
  value    `(a, b)`        -> `.(a, b)`              (named uses `=`)
  typed    (new)           -> `Tuple(A, B).(a, b)`   (like `Point.{...}`)
  failable `-> (T, !)`     -> `-> T !`
           `-> (T1, T2, !)`-> `-> Tuple(T1, T2) !`   (channel outside Tuple)

Bare `(...)` is now grouping only, everywhere; a comma in bare parens is a
hard error with a migration hint. Grouping, function types `(A, B) -> R`,
param lists, lambdas, and match bindings are unaffected.

`Tuple(...)` is strictly a TYPE in every position (including `size_of` /
`type_info` args); a tuple VALUE comes only from `.(...)` (anonymous) or
`Tuple(...).(...)` (explicitly typed). A bare `Tuple(1, 2)` is a tuple
type with non-type elements -> rejected.

The ~110 tuple-bearing corpus files were migrated with a one-shot
AST-aware migrator (the `sx migrate` tool from the prior commit, removed
here). New examples: 0130 (new syntax), 0131 (typed construction), 1060
(named-tuple failable return). 1116 golden updated for the new hint text.

examples/errors/1018-errors-multi-value-failable.sx

@@ -12,37 +12,37 @@
 
 E :: error { Bad, Empty }
 
-parse :: (n: i32) -> (i32, i32, !E) {
+parse :: (n: i32) -> Tuple(i32, i32) !E {
     if n < 0  { raise error.Bad; }
     if n == 0 { raise error.Empty; }
-    return (n * 2, n + 1);            // success → {n*2, n+1, 0}
+    return .(n * 2, n + 1);            // success → {n*2, n+1, 0}
 }
 
 // Multi-value `try` in a multi-value caller — propagates {undef, undef, tag}.
-inc :: (n: i32) -> (i32, i32, !E) {
+inc :: (n: i32) -> Tuple(i32, i32) !E {
     v, b := try parse(n);
-    return (v + 1, b + 1);
+    return .(v + 1, b + 1);
 }
 
 // Multi-value `catch`, bare-expression tuple fallback (absorbs the failure).
 safe :: (n: i32) -> i32 {
-    v, b := parse(n) catch (e) (40, 50);
+    v, b := parse(n) catch (e) .(40, 50);
     return v + b;
 }
 
 // Multi-value `catch` match-body — per-tag dispatch, each arm a value-tuple.
 classify :: (n: i32) -> i32 {
     v, b := parse(n) catch (e) == {
-        case .Bad:   (1, 1);
-        case .Empty: (2, 2);
-        else:        (9, 9);
+        case .Bad:   .(1, 1);
+        case .Empty: .(2, 2);
+        else:        .(9, 9);
     };
     return v + b;
 }
 
 // Multi-value `or (tuple)` value-terminator (absorbs the failure).
 ortest :: (n: i32) -> i32 {
-    v, b := parse(n) or (7, 8);
+    v, b := parse(n) or .(7, 8);
     return v + b;
 }